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Probably Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 14 Jul 2003 10:50:17
In Reply to: Alternator problems (or possibly not...), Rod M, Mon, 14 Jul 2003 09:25:58 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Let me see if I get this straight-
Battery voltage never makes it above 12.1 volts. Battery light is always on. I assume this is when the engine is running. I'll also assume that you've actually measured the battery voltage with a voltmeter to back up the EDU reading.
The battery voltage should be around 12.6 volts with the engine off, low 12's running without the alternator working. Alternator output should be in the 13's.
A few possibilities. First, disconnect the thin wire (usually yellow or green) going to the back of the alternator, and turn the car to ON. The engine doesn't have to be running. If the BAT light comes on, you've found you're problem - there is a short in that wire. The high side of that light bulb is connected to the battery (via the ignition switch) - the other side of the light is that yellow (or green) thin wire running to the alternator. The idea is that wire supplies current to the alternator field coils. This looks like a ground when the alternator isn't putting out power, so the circuit is complete and the light is ON. Once the alternator gets going and starts putting out voltage, then both side of the bulb are at battery voltage, and the light goes out. IF that wire is shorted to ground, (1) the BAT light will be on, and (2) the alternator field coils will never get energized. You can spin the alternator from now 'till doomsday and it'll never put out any current, yet it will test fine out of the car.
Other possibilities - the shop did a lousy job of testing. I assume they'll find a bad diode in the alternator. I'm not sure how to work this, other than asking the shop what types of things they can find, and if diode problems aren't on the list, find another shop. Also ask if they load test the alternator - electrical devices act very differently loaded or unloaded.
I'll assume that the belt is properly tensioned and that the alternator pulley is spinning. Just have to ask.
I don't suspect battery problems. The purpose of the battery is to provide enough juice to start the car. The purpose of the alternator is to provide all the power requirements of the car, and to have enough left over to recharge the battery after its effort to start the car. In order to do this, the alternator has to provide a higher voltage than the battery (so that current flows into the battery, charging it). That means an alternator voltage of >12.6, preferably 13.5 or more. A bad battery won't cause the alternator not to work, not unless there is a major short circuit in the battery to the point where the alternator can't source current into it. And in that case, the battery wouldn't have the OOmmph to turn the engine over. Low charging voltages are alternator, not battery, issues.
As usual, make sure all cable connections are good - same with grounds. But I'd go after a shorted wire first.
posted by 192.249....
, Mon, 14 Jul 2003 10:50:17 <-- Viewing This Message, Tue, 15 Jul 2003 09:59:35, Tue, 15 Jul 2003 14:29:38
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